The General & Special Senses

Chapter 17

Introduction

• Senses – our perception of what is “out there”

• 2 groups– General senses– Special senses

General Senses

• Includes senses that are skin or kinesthetic– Temperature, pressure, touch, pain, vibration,

proprioception

• Pass information along the spinal nerves and pathways to the somatosensory cortex

Special Senses

• Olfaction, gustation, equilibrium, hearing, & vision

• Found within complex sense organs

• Pass information along the cranial nerves to specific areas of the cerebral cortex.

Receptors

• Sensory receptors are transducers – Change stimuli into electro-chemical impulses – Specific receptors can transduce only certain

types of stimuli

• Two general types– Exteroceptors– Interoceptors

Interpretation of Sensory Information

• Occurs in cerebral cortex

• Depends on the area of the cerebral cortex that receives the information

• Also depends on the sequence of neurons carrying the information

Central Processing and Adaptation

• Adaptation – the loss of sensitivity after continuous stimulation– Tonic receptors are always active– Phasic receptors only relay changes in the

conditions they are monitoring

• Role – prevents brain from being overloaded with unimportant information

Receptors of the General Senses

Nociceptors

• Detect pain– Referred pain– Phantom pain

Mechanoreceptors

• Respond to pressure & touch– Tactile receptors– Baroreceptors– Proprioreceptors– Thermoreceptors

Tactile Receptors• Found in the dermis

Baroreceptors• Monitor changes in pressure

Chemoreceptors• Detect chemicals in solution

– Blood composition

The Special Senses

Olfaction (the nose)• Olfactory receptors

– Can detect at least 50 different primary smells

– Located in the epithelium of roof of nasal cavity

Olfactory Receptors

• Molecules dissolve in the mucus or lipids of the epithelium

• Olfactory neurons pass through the cribiform plate and synapse in the olfactory bulb

• Olfactory tracts go directly to the cerebral cortex

Gustation (the tongue)

• Taste receptors are in the taste buds

• 4 primary tastes– Sweet, sour, salty,

bitter• Located in papillae on

the surface of the tongue• Contain the gustatory

receptors– Molecules dissolve in

saliva

Pathway of Gustatory Sense

• 3 cranial nerves relay sensory impulses to the cerebral cortex– Facial,

glossopharyngeal, vagus

– All pass through the medulla & thalamus

Equilibrium & Hearing (the ear)

• External ear– The auricle directs sound waves into the external

auditory meatus to the tympanic membrane

The Middle Ear• Located in the temporal bone• Contains the auditory ossicles

– Malleus

– Incus– stapes

• Connected to throat by the eustachian tube

The Inner Ear

• Located in the temporal bone• Separated from the middle ear by the oval window

The Inner Ear

• Consists of a series of canals called the bony labyrinth– Lined by a

membranous labyrinth filled with endolymph

– Perilymph flows between the bony and membranous labyrinth

The Inner Ear

• The bony labyrinth is divided into 3 areas– Vestibule contains

• Saccule & utricle– Semicircular canals

• Each has an ampulla

– Cochlea contains• Organ of Corti

The Utricle & Saccule

• Detects static position• Hair cells are

embedded in a gelatinous material

Otoliths at Work• Otoliths are

balanced on top of gelatinous material– Slide when head

tips

– Bend hairs

– Generates nerve impulse

The Semicircular Canals• Detect dynamic

balance• Arranged at right

angles to each other• In the ampula, hair

cells are embedded in gelatinous material (cupula) with fluid over it

• Movement of head– Bends the hairs – Creates nerve

impulses

Semicircular Canals at Work

The Cochlea

• Divided into 3 tunnels by the vestibular and basilar membranes– Scala vestibuli ends

in the oval window– Cochlear duct

contains the organ of Corti

– Scala tympani ends in the round window

Cochlear Chambers

The Organ of Corti

• Consists of hair cells on the basilar membrane

• Tips of hairs touch the tectorial membrane

• Basement membrane vibrates– Hair cells bend

– Sends a nerve impulse

Pathway of Auditory Sense

Summary of Hearing

• Sound waves enter the external auditory meatus• Tympanic membrane vibrates• Auditory ossicles vibrate• Oval window vibrates• Perilymph in scala vestibuli & scala tympani moves• Basilar membrane moves• Hairs rub against the tectorial membrane• Nerve impulse is sent along the vestibulocochlear

nerve to the brain

Vision (the eye) – Accessory Structures• Eyelids protect the eye

– Conjunctiva lines the eyelid

• Lacrimal apparatus– Lacrimal gland produces

tears– Lacrimal canals drain

tears into lacrimal sacs– Nasolacrimal duct drains

into the nasal cavity• Extrinsic muscles move

the eyeball

Structure of the Eye – 3 Tunics

• Fibrous tunic

– Includes cornea & sclera• Vascular tunic

– Includes choroid coat, ciliary body, lens, iris & pupil

• Neural tunic (retina)– Contains photoreceptors

• Rods & cones– Includes optic disc,

macula lutea & fovea centralis

Human Anatomy, 3rd editionPrentice Hall, © 2001

Figure 18-22c

Photo of Posterior Eye

The Cavities of the Eye

• The lens separates the interior of the eye into 2 cavities– Anterior cavity

• Contains aqueous humor

• Glaucoma

– Posterior cavity

• Contains vitreous humor

The Vascular Tunic

• Contains many blood vessels & nerves

• The iris controls the size of the pupil

• Suspensory ligaments attach the lens to the ciliary body– Controls the shape

of the lens

The Retina

• Cones allow for sharp color vision in bright light– Contain pigments– Macula lutea – Fovea centralis

• Rods provide for vision in dim light– Contain the pigment

rhodopsin– Most dense at

periphery of retina

Pathway of Vision Sense

Summary of Vision

• Light rays enters through the pupil

• Light rays cross in the lens

• Retina receives reversed & upside down image

• Rods & cones are stimulated

• Optic nerve carries impulse to the brain

Abnormal Vision

• Myopia

• Hyperopia

• Presbyopia

• Astigmatism